1. Field of the Invention
The present invention relates to a planar illumination device in which laser light is used as a light source and a liquid crystal device using the planar illumination device, and more particularly to a planar illumination device that is thin and applicable to a large screen display and that has a high light utilization efficiency, and a liquid crystal device using the planar illumination device.
2. Description of the Background Art
Generally, as a liquid crystal device used for a display panel and the like, a planar illumination device using a light source such as an electric discharge tube or a light-emitting diode (LED) as backlight illumination is employed. In recent years, when the planar illumination device is used in a large display and the like, light having high brightness and high monochromaticity is required. Therefore, a planar illumination device using a laser light source has been considered.
Such a display panel requires a technique to reduce luminance unevenness on the entire surface of the display panel to thereby make uniform the brightness thereof, or a technique to improve the light utilization efficiency to thereby reduce power consumption. Also, in recent years, along with the upsizing of the display panel, there is an increased demand to construct the display panel to be thinner for usage as a wall-mounted television or the like.
As a method of reducing the thickness of a planar illumination device used for a display panel, there is a technique called an edge light system. In the edge light system, light is incident from aside surface of a light guide plate, and the incident light is deflected toward a main surface of the light guide plate by using deflection ridges or a diffusion member provided on a bottom surface of the light guide plate so as to be radiated from the main surface of the light guide plate, thereby obtaining planar illumination. The edge light system has a characteristic that a light source is arranged in a planar manner, i.e., a planar illumination device can be structured to be thinner than the case where a so-called direct-under system is used.
In the edge light system, as a structure which realizes a large screen and high brightness, a structure in which light is incident from side surfaces on two sides of a light guide plate has been proposed. As disclosed in Japanese Laid-Open Patent Publication No. 2001-228477 (hereinafter, referred to as patent document 1), for example, a light guide plate is formed such that a surface on the side of a liquid display panel (main surface) is a flat surface, and a surface opposite to the main surface, on which deflection ridges are formed (bottom surface) is gradually thinned from both side surfaces from which light is incident toward a center thereof (i.e., the bottom surface is formed into an inverted V-shape). With such a structure, the weight of the light guide plate also can be reduced.
Furthermore, a light source having a high output is required in order to respond to a screen with an increased size. This can be realized by using a high output laser light source. As a structure using a laser light source, a structure proposed in Japanese Laid-Open Patent Publication No. 11-271767 (hereinafter, referred to as patent document 2) can be used, for example. Patent document 2 proposes the structure in which LEDs are disposed on two sides of a rod-shaped light guide member. When the LEDs are replaced with high output laser light sources in this structure, however, it is possible to realize a high output linear light source, and therefore this structure is applicable to a large screen display.
In order to realize a thin and large screen display, it may be suitable to combine the structure of the light guide plate proposed in patent document 1 with a high output laser light source. Furthermore, the structure proposed in patent document 2 can be used to convert light from a laser light source into a linear light source.
However, even with a structure obtained by combining patent document 1 with patent document 2, there has been a problem in that a portion of light incident from side surfaces on two sides of a light guide plate reaches the side surface on an opposite side from which the light is incident, and then the portion of light is to be radiated therefrom, thereby reducing a light utilization efficiency. Hereinafter, this problem will be described.
FIG. 12 is a diagram illustrating a schematic structure of a conventional planar illumination device 100 having a structure obtained by combining patent document 1 with patent document 2. Particularly, FIG. 12(a) is a schematic view of the planar illumination device 100 as viewed from a top surface thereof. FIG. 12(b) is a schematic view illustrating a structure of a main portion included in a side surface of the planar illumination device 100 as viewed from an arrow E of FIG. 12(a).
In FIGS. 12(a) and (b), the planar illumination device 100 comprises laser light sources 101 to 104, rod-shaped light guide members 105 and 106, and a light guide plate 107. The light guide plate 107 is formed such that a main surface 107a from which light is incident is a flat surface and a bottom surface 107b is gradually thinned toward a center thereof. On the bottom surface 107b, deflection ridges for deflecting the light incident on the light guide plate 107 toward the main surface 107a are formed.
In the planar illumination device 100 structured in such a manner as described above, laser light radiated from laser light sources 101, 102, 103 and 104 is converted into linear light by light guide members 105 and 106, so as to be incident on the light guide plate 107. For conciseness, in FIG. 12(b), only laser light radiated from the light guide member 106 is indicated by a dotted line, and laser light radiated from the light guide member 105 is omitted. As shown in FIG. 12(b), among light incident on the light guide plate 107, a portion of luminous flux 108 passes through the light guide plate 107 so as to reach a side surface on an opposite side, thereby causing a light quantity loss.